Once a custom font is cached to disk, it starts failing to render until the page is refreshed.
<rdar://problem/13622998> and https://bugs.webkit.org/show_bug.cgi?id=115131

Reviewed by Alexey Proskuryakov.

No new tests (Not a tested config, nor are disk cache issues currently testable).

* loader/cache/CachedResource.h:
(WebCore::CachedResource:: mayTryReplaceEncodedData): Allow subclasses to refuse encoded data replacement.

* loader/cache/CachedResource.cpp:
(WebCore::CachedResource::tryReplaceEncodedData): Only try if shouldTryReplaceEncodedData() is true.

* loader/cache/CachedFont.cpp:
(WebCore::CachedFont::CachedFont):
(WebCore::CachedFont::ensureCustomFontData):
(WebCore::CachedFont::mayTryReplaceEncodedData): Return false if the custom font data has ever been created.
* loader/cache/CachedFont.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149070 268f45cc-cd09-0410-ab3c-d52691b4dbfc